What are their issues? Medusas are the typical 36mm in diameter, so that should be good. It would have to be the length if anything. I can understand Neus having trouble since the 15xx series is ~44mm in diameter...
I don't think it will fit because the front of the medusa is not flat, the bearing sticks out some from the front of the motor. I guess you could always drill the hole out a bit larger in the mount.
I think they have a 25mm spacing. If the front bearing is the problem though, just get Mikes plate that goes on the front. Also, if the Losi mount is for 4mm screws, you can tap the cover for 4mm or Mike could probably make you one like that as well.
The losi mount is pretty thick. I don't know how long the shaft sticks out of the end of a medusa, but it would probably be pretty close to making the shaft too short if you spaced it back to make up for the bearing. I have a cc2200, which has a pretty long shaft, and the pinion doesn't really go on it all that far. I just use 3mm screws, and have had no problems. The pinions suck that come with the kit, the mount is thick, it comes with a metal spur as well instead of them giving you the plastic one? I guess the radio box is a nice touch. I would just buy the rcm mount, losi tray, and radio box, if I had it to do over again.